设置UIView的mask约束边界

UIEdgeInsets stretchInsets = UIEdgeInsetsMake(32, 40, 10, 40); UIImage* backImage = isOutgoing ? [UIImage imageNamed:@"bubble_right.png"] : [UIImage imageNamed:@"bubble_left.png"]; backImage = [backImage resizableImageWithCapInsets:stretchInsets resizingMode:UIImageResizingModeStretch]; //设置图片的 mask layer CALayer* layer = [CALayer new]; layer.contents = (__bridge id _Nullable)(backImage.CGImage); CGRect r = [self CGRectCenterRectForResizableImage:backImage]; layer.contentsCenter = r; layer.frame = CGRectMake(0, 0, fixSize.width, fixSize.height); layer.contentsScale = [UIScreen mainScreen].scale; layer.opacity = 1; imageView.layer.mask = layer; imageView.layer.masksToBounds = YES;

这里通过设置 UIImageView的 layer.mask 来设置约束，这里通过一张图片，去图片的边缘部分组成一个path，path以内的部分将会显示出来，path以外的部分将不会显示，backImage 决定了path的形状！ 当然了这里还可以设置其它的layer作为uiview的mask。

附这里用到的另外一个方法：

- (CGRect) CGRectCenterRectForResizableImage:(UIImage *) image { return CGRectMake( image.capInsets.left / image.size.width, image.capInsets.top / image.size.height, (image.size.width - image.capInsets.right - image.capInsets.left) / image.size.width, (image.size.height - image.capInsets.bottom - image.capInsets.top) / image.size.height ); }